Developing a Power Supply, input voltage
range 90-230Vrms with 20 watt power, Power Factor Controlled. Application of this power supply is not important, board name and part names not important. This application note is not a
instruction-guide or a part-guide to rebuild this circuit.

Better
visit IC manufactorer websites when searching for a circuit, don`t use
your time to search here.

The purpose of this application note
is to give a short overview how many measurements are necessary to get a final circuit design.

Safety Considerations: working
on 400 Volt AC is very dangerous for your life, do this electronic work
only when you have the skills, safety tools and when you are professional trained to
work with higher voltage. Use isolating safety transformers. Don´t work allone.

Oscilloscopes

Why
using here an older type of digital scope? When developing an unknown
evaluation board with 400V, it is a potential risk to destroy
the test equipment. Especially the first hours when getting experience
with the board are the worst. One known problem, accidential misuse
measuring on the wrong potentials on different grounds. Using safety
transformers is an additional must.

Oscilloscope here has a max. 250Vpk
input voltage range, 1:10 and 1:100 probes can extend the range.
For me a risk using more expensive higher bandwidth models
having only 100Vpk. input
voltage
range - avoiding tears of pain after an accidential misuse.

This older type, year 1997 has enough performance for such a development, easy
to operate and trustful measurement results, it is still an excellent oscilloscope.

Load 16 Watt @230Vrms - Power-ON

Current peak at t=0 comes
from small filter foil capacitors in the primary EMC filter. During
start-up input current remains low. Output voltage settle to a
stable value, overshooting to 26V.

Load 16 Watt @230Vrms - Power-ON Input Current Peak

Figure 6

Channel 2 = Input Current of EMC foil capacitor

First
current peak at t=0 comes from filter foil capacitors in the
primary EMC filter. Input current should be considered when driven by
small semiconductors, should always remain within the drivers safe
operating area.

Peak current depends on phase angle of the input
sine wave. Catching a trigger at >1A, ten times Power-ON applied. Finding
the maximum possible peak current requires a equipment, switching ON under defined phase angle.

Part 2

Increasing the Output Voltage to 28-29VDC, application changed

Output Load 24 Watt @230Vrms - settled output

Power Transformer and
secondary side rectifier diode runs too HOT under 24W constant load,
these parts can not be used for a permanent 24W load, possible for a
short-time.

What is short time? Can not answer this question, depends on
environmental temperature, thermal mass of transformer and diode heat
sink - not a question of today.

Figure 11
Switching off line power,
fine Power-Off

Figure 12
Remove 24W load, output jumps up to 32V, settle later to 30V.
Depends on application if this voltage jump is acceptable.

24W Load needs at least a 150VAC rms input, lower voltage require another current limit or less output power.

EMC Problem with this Evaluation Board

Figure 13

When running the
Evaluation with 24W load there is an EMC problem. The radio on the
photo is a digital radio with DAB+ standard. One station
"xxx" stops receiving when the Evalboard is loaded with overpowered 24W.

Transformer has been exchanged, additional using too long transformer wires, can be a reason.

When increasing the distance between radio and Evaluation Board (antenna up, like in the photo), the
radio program received clearly. Before the antenna was hanging down, this was too near.

Checking Emitted Air Spectrum

Figure 14

Signal Analyzer

Span 9kHz to 13GHz, very sensitive instrument

Broadband antenna, usable as indicator from 50MHz to 2GHz, linear range from 100MHz to 1GHz

When
developing a circuit with fast changing currents und voltages always
use a spectrum analyzer to observe the circuit, beginning
from the first hour of the development.

Part 3

Shorting the 5V Auxiliary Winding

Figure 17

Board has
a 28V/24W power winding and 5V/4W auxiliary winding. In Figure 17 there
is no load on the
power winding and a short in the 4W aux. winding. Question, does the
controller detect the short cut? Yes, controller detects a short-cut
via the second primary winding and shut off. Results in an ON-OFF
oscillation. Transformer windung runs not hot, no other part runs hot.
Controller has a fine short-cut behaviour.

5V winding is
short-cut protected but not overlaod protected. This transformer and
circuit can not be used for the target application with the requirement
of safe permanent overload conditions.

Application requires another
type of transformer with a single secondary winding only. 5V auxiliary
voltage must generated by a DCDC converter with internal overload
protection, converter driven by 24V.

Testing another Power Supply for EMC Comparison

Figure 19

Simple EMC test of
a worldwide input 120W power-supply from a flatscreen television,
made year 2014.

Figure 20

Air spectrum with powered-OFF TV power supply, line cable removed from wall
socket. Visible the local FM stations and DAB+
stations. Span 9kHz-300MHz, sweeptime 10ms.

Figure 21

Powered-ON TV power supply. Span 9kHz-300MHz, sweeptime
10ms.

Primary Current in the Transformer

Figure 22

Primary winding current

Off-resonant controller, fast current change. During MOSFET
Off current stops fast, resulting in LC-oscillation of primary
winding leakage inductivity with MOSFET capacitance.

Power Factor Quality

Figure 24

Line current and
line voltage under full load. Power Controller ensures that the load
behaviours like an ohmic resistor, line current and line voltage
should follow each other in phase, the waveform should look the same.

Figure 25

Line
current and line voltage multiplied to power. With an ideal PFC
controller and ideal sinewave voltage the power signal should be a 100 Hz
x²signal. Power Factor is distorted in the maximum power
area.

Parameter measured on Waveform A:

pkpk(A) 58W

mean (A) 24W

rms (A) 32W

Measuring the efficiency of this PSU, 8-bit
scope a wrong tool, requires a power-analyzer or rms precision
multimeters, Two DMM on primary and two DMM on
secondary side.

For a final PCB version this small SMD transistor would require a
PCB-layout heatsink area large as possible. Enough thermal vias to the
inner-layers highly recommended.

The heat sink is connected to the Drain potential. Drain-to-Source
Capacitance reacts with the parasitic transformer incductance as LC
oscillator used in this ZCS Zero-Current-Switching system. In the first
"valley" switch the transistor.

Figure 30

Fly-Back,
results in high Drain-Source voltages. When driving the
Evalboard with 280VAC rms the Drain voltage increase up to 700V. MOSFET transistor has a 800V specification.

700V is large and a dangerous number. When doing the layout this high
voltage has to be considered in surface-leakage and isolating distance.

Also when measuring such a high voltage use a 100:1 probe, the used
probe has a 1,5kV specifiaction. Don`t use 10:1 probes, many of
them are not specified for 700V, can damage oscilloscope
input.

Drain voltage oscillates with approx. 450ns, 2.2MHz

Transformer winding leakage inductance 35µH

Drain source capacitance, external smd 220pF

LC resonant frequency: 1/2*PI*sqrt(L*C) approx. 2 MHz

Theorie and measurement - same result.

Figure 31

Waveform with respect to the Gate voltage (Ch.4)

Figure 32

Before in Figure
30 the drain current is not correct measured, the bandwidth of the
power current probe (max. 150A, 120kHz) is too low. Therefore for the
fast changing drain current a 50MHz type.

Figure 33

Fast drain current changes measured with 50MHz probe. Drain current (Ch.1) and Drain voltage (Ch.3) are now exactly in phase.

step 1.----------------------Gate ON-------------------------------------
When switching ON the Gate, current in the transformer starts
with a ramp. Current increase linear, a good
message, the inductance of the transformer remains constant even under
higher currents, core is not overloaded. An overloaded core
would show an non-linear ramp with a fast slewing ramp under high currents.

step 2.---------------------Gate OFF-------------------------------------
After reaching a maximum current level (detected by the controller) the
Gate switch OFF and stops continuing charging the core with
magnetic energy. Current in the winding stops very fast, Gate goes low with fastest slew-rate possible by the controller.

For the gate discharge there is no smd resistor in the gate, only a small signal diode, fast OFF.For the gate charge there is a 220ohm resistor in the gate, resulting in an slower ON.

Gate OFF must be done as fast as possible to keep the MOSFET switching
loss to a moderate level. Consider when switching OFF the MOSFET the
Drain voltage is low, but the Drain current is at a maximum.

OFF switching-power = draincurrent*drainvoltage

Disadvantage for the fast OFF switching, increased EMC emission:
Drain current changes very fast, large high H-field emission.
Drain voltage changes very fast, large E-field emission.

step 4.---------------------Demagnization-------------------------------
stored magnetic energy in the transformer core runs empty (energy transfer to secondary side).
When energy transfer stops (stored magnetic energy empty), voltage at the primary winding increase with u=L*di/dt. An
increasing reversed voltage across the primary winding reduces
automatically the drain voltage. This automatically decreased drain
voltage can be observed as the slowly falling drain voltage ramp. The
controller does the same, measure and search for a moderate -du/dt
drain voltage. After detecting this demagnization point the controller
will ON the MOSFET again.

step 5. ------------------------ON--------------------------------------
exactly after core demagnizited (empty magnetic energy)
it is a nice moment to power ON the core again. At this moment no
current is flowing in the primary winding and no drain voltage (voltage
across coil), it is an ideal point, less MOSFET
switching losses. Because drain voltage and current is low, the gate can
be switched ON slowly, thats why there is a Gate-ON 220 Ohm
resistor, reducing the MOSFET ON time. Swichting ON creates not much
MOSFET switching losses and not much EMC-emissions, this is an advantage.

MOSFET Power OFF creates much EMC-emission and much thermal Switching-losses.MOSFET Power ON creates less EMC-emission and less thermal Switching-losses.

ZCS (Zero Current Switching) method:approx. 50% of the EMC-emission and thermal Switching-loss could be reduced under MOSFET ON.Remaining 50% EMC-emission and thermal Switching-loss during MOSFET OFF can not be reduced.

E-field and H-field emission can be only reduced, using a larger slower switching MOSFET
with a larger thermal heatsink and less power efficiency.

It is a question of the application what is more important: less EMC-emission or a better power efficiency.